Tensile Testing and Non-destructive Evaluation Scanning of Varied Ply CFRP Laminates with Embedded Magnetostrictive Particles

摘要

One method for Nondestructive Evaluation (NDE) of Carbon Fiber Reinforced Polymers (CFRP) is the integration of magnetostrictive particles (MSP) into CFRP laminates. In order to ensure that integration of MSP would be a feasible method of nondestructive evaluation, the structural intrusiveness of the particles must be investigated. Since one of the major benefits of CFRP is its strength to weight ratio, the inclusion of a foreign material must be minimally intrusive on the material properties of the laminate. This paper details several analyses meant to quantify the affect of integrating MSP into unidirectional CFRP laminates on the quasi-static tensile properties. First, the material properties of both the MSP and CFRP laminate were used in conjunction with the constitutive equations to estimate the ultimate tensile strength of symmetric laminates with and without MSP. Once a prediction of the failure stress had been determined, laminates were fabricated and tested under quasi-static tension. Finally, the experimental tests were correlated with a Finite Element Model. The laminates tested ranged from two plies to ten plies with a single layer of MSP embedded in the center of laminates. The results from experimental testing of carbon fiber-reinforced polymer composite beams with and without a layer of magnetostrictive particles showed that the particle layer was minimally intrusive on the quasi-static tension properties of the beam. Analysis of the results revealed that the addition of a layer of MSP caused a slight increase in the ultimate tensile strength of the beams, while the modulus saw an equivalent drop. Based on the number of samples tested, the amount of change seen in both the tensile strength and the modulus was statistically negligible. An investigation using scanning electron microscopy shed light on the change in the material properties. It was seen that during the curing process the epoxy matrix flowed into the voids between particles in the particle layer, which caused an increase in the fiber volume in the ply region. Previous research has shown that an increase in fiber volume in fiber reinforced composites leads to an improvement in ultimate tensile strength. This is because tensile strength is a fiber-dominated property. The microscopy analysis also showed that while there was flow over of epoxy resin into the particle layer, there were still large voids present. The presence of voids inside a laminate would reduce the incubation time for matrix cracks, limiting the matrix's ability to transfer stress between fibers and reducing the stiffness of the laminate. The results from the analytical model proved to overestimate both the ultimate tensile strength and the modulus. The analytical model was based on equations and material properties for ideal laminates. Experimental testing tends to encounter issues such as material flaws, fabrication inconsistencies, and testing inconsistencies, which cause the results to deviate from the analytical model. While the experimental results and analytical model did not completely agree, the difference was not an order of magnitude in size. On the other hand, the results from the finite element analysis were in close agreement with the experimental data. For laminates with ply numbers of fewer than the data was statistically equivalent to the FEA model, with the higher laminate numbers seeing a slight overestimation in the ultimate tensile strength. The overestimation was likely the result of a difference in the theoretical thickness and fabricated thickness of the laminates. The thickness used in the finite element model was based on the manufacturer's recommended thickness, which, due to epoxy flow out, would differ from the thickness of fabricated laminates. In lower ply laminates the thickness difference would not be pronounced, but in laminates with more plies the difference is compounded and would have a greater affect. While this research shows that the addition of a layer of magnetostrictive particles does not have an appreciable affect on the quasi-static tension properties of fiber reinforce composites, future research will have to be conducted to investigate it's affects in different loading scenarios. The results gained from this research are promising for the performance of these laminates under compressive loading. A fiber reinforced laminate's compressive properties, like its tensile properties, are fiber dominated. Since the addition of a particle layer resulted in an increased fiber volume in the ply region, it is postulated that compression testing will also reveal the particle layer is minimally intrusive. The results of the microscopy study reflect poorly on the performance of laminates under the action of bending forces. Bending of beams involves the upper and lower surfaces of the laminate being under the action of opposing stress states. One surface is under the action of compressive stress, while the other is under the action of a tensile stress and the center of the beam is integral in the transfer of these two stress states. The microscopy study revealed that the particle layer, located in the center of the beam, is riddled with voids. This presence of voids would inhibit the layer's ability to transfer the stress effectively between the two surfaces. It is believed that a laminate containing a layer of magnetostrictive particles under the action of a bending stress would delaminate along the centerline, resulting in premature failure. Additionally, in this paper, experimental and numerical results of damage sensing tests using magnetostrictive particles embedded in fiber reinforced polymer laminates, are presented. For the experimental results, carbon fiber reinforced polymer (CFRP) laminates (Hexcel AS4/3501-6) are embedded with Terfenol-d particles and the ply count is varied to observe the change in the sensing. Sensing is observed using a non-contacting magnetostrictive strain sensor setup. The sensing parameter observed is the voltage induced in the secondary circuit. Two of the three batches presented have laminates that are embedded with .5"x.5", release agent coated patches that prevent bonding between the Terfenol-d and the CFRP layer. The laminate ply count ranges from 2-14 unidirectional plies. Two fabrication methods are used to distribute the particles in the laminate. The experimental results from the three batches reveal that the fabrication technique has a significant effect on the sensing signal. The effect of particle accumulation in the magnetic flux path significantly affects the sensing signal and makes the presence of a delamination difficult to assess. The experiments also show that when the ply count is varied, there is not much variation in the sensing signal. For the numerical simulation, finite element models are constructed to analyze the magneto-mechanical interaction in a laminate embedded with terfenol-D (TD) for sensing purposes. The model examines the mechanical parameters that affect sensing by observing the parameters in the Terfenol-d layer around a delamination. The results of the model show that the magnetic field produces a local change in strain and stress in the region of the delamination. The strain in the delamination region is greater than the strain in the bonded regions. The stress in the delamination region is lower than the stress in the bonded regions. For the magnetic parameters, a constant magnetic permeability was used in the constitutive magnetostriction equation, which inhibits the magnetic flux density from varying as a function of stress/strain. The data shows that the distribution method used to distribute the particles have a dominant effect on the sensing signal. Attempts were made to refine the distribution method, but it is highly suggested that an automated method be used to distribute these particles. The effect of ply variation cannot be distinguished due to inconsistencies in the sensing signal. The delamination also could not be clearly distinguished from a change in particle density. Future work will focus on finding new ways to distribute particles and differentiate particle clumping from a delamination. The multiphysics FEA model reveals that there is a drop in stress and strain in the region of the delamination for a laminate that has a delamination against the magnetostrictive layer.